1. Field of the Invention
The present invention relates to a technique for providing a plurality of different frame rates for use in time-division multiple access (TDMA) communication system and, more particularly, to a technique which uses the well-known time-division multiple access mode while still providing a plurality of different frame rates which is dependent on the traffic demands between the various pairs of remote, spaced-apart, ground stations making up the communication system to provide efficient access therebetween.
2. Description of the Prior Art
In a time-division multiple access (TDMA) communication system, a transmitting station or area normally communicates with a plurality of remote, spaced-apart, receiving stations or areas by sequentially accessing each of the receiving stations or areas before repeating the sequence. The period of transmission to each receiving station or area is generally known as a transmission burst and in general the sequence of transmission bursts is synchronized in a TDMA frame period and each of the transmitting station and receiving stations are synchronized to enable reception of the associated transmission bursts. In this regard see, for example, U.S. Pat. No. 3,772,475 issued to A. Loffreda on Nov. 13, 1973 and in particular FIG. 1 and the associated description therein.
It is generally found that the traffic demands between a transmitting station or area and each of the receiving stations or areas is not equal and to compensate for such diverse traffic demands prior art TDMA systems have used longer or shorter transmission bursts in a direct relationship to such traffic demands. In this regard see, for example, U.S. Pat. Nos. 3,711,855 issued to W. G. Schmidt et al on Jan. 16, 1973, and especially FIGS. 4A and 4B thereof where two bursts are used for high traffic stations; 3,778,715 issued to W. G. Schmidt et al on Dec. 11, 1973 and especially FIG. 3A thereof; 3,789,142 issued to N. Shimasaki et al on Jan. 29, 1974, especially FIGS. 8A-8C; and Re. 28,577 issued to W. G. Schmidt on Oct. 21, 1975, especially FIGS. 1 and 2.
The TDMA format is primarily used in multiple beam satellite communication systems and in such systems it is desirable from a weight and reliability consideration to have a single transponder associated with each beam which is operating at the maximum channel bit rate. For example, at 12/14 GHz the transponder bandwidth may be 500 MHz and can comfortably support 4 .phi. PSK modulation at 600 Mbs/sec (300 Mbauds/sec). For a channel transmission at such a high rate which is to be accessed by perhaps hundreds of stations in that beam, it is imperative to have an efficient access scheme. Time-division-multiple access provides such a solution if the number of accesses is small. When the number of accesses is large, the overhead associated with each transmission burst starts to cut into the system efficiency and longer frames must be used. Associated with the use of longer frames is the problem that buffer (high speed) requirements correspondingly increase. Thus, in the prior systems, the problem exists that for system efficiency, longer frames should be used, yet for buffer size considerations, shorter frames should be used.